Origin of the largest South American transcontinental water divide.

Interbasin arches between hydrographic systems have a heterogeneous geological origin, forming under the influence of several different geomorphological processes. Independent of the underlying processes, these arches compartmentalize present-day river basins, encompassing different water chemistries, habitat types, soil domains, potential energy and, on a geological/evolutionary time scale, aquatic life varieties in the ecosystem. Through most of its length, the water divide between the Amazonian, Paraná-Paraguay, and São Francisco river basins in central South America coincides with an Upper Cretaceous intracontinental igneous alkaline province. This magmatism, independent of its nature, caused intense crustal uplift and influenced hydrological networks at different scales: from continental-scale crustal doming to continental break-up, and finally to local-scale phenomena. The available ages for alkaline rocks indicate a well-defined time-interval between 72.4 to 91 Ma (concentrated between 76 and 88 Ma) period of uplift that contributed to large-scale drainage compartmentalization in the region. Here we show that uplift associated with intrusive magmatism explains the origin and maintenance of the divide between the Amazonian, Paraná-Paraguay, and São Francisco river basins.

A new Otocinclus (Siluriformes: Loricariidae: Hypoptopomatinae) from the Rio Juruena Basin, central Brazil.

A new species of Otocinclus (Loricariidae) is described from the Rio Juruena, a right bank tributary of the Rio Tapajós system in Mato Grosso State, central Brazil. The new taxon can be distinguished from its congeners by the possession of an ocular operculum and by a complete lateral line. Additionally, the new species is distinguished by having a dorsal trunk coloration composed of a set of distinct oval dark blotches and by a caudal spot.

A new species of the lowland Oligosarcus Günther species group (Teleostei: Ostariophysi: Characidae)

A new characid species is described from the rio Jacuí basin, a coastal drainage from southern Brazil. Itis morphologically most similar to its sympatric congeners Oligosarcus jenynsii and O. jacuiensis, but can be distinguished from both species in having the interorbital region wider and the pectoral fin shorter and additionally from O. jenynsii in having a smaller orbital diameter.

Uma nova espécie de caracídeo é descrita da bacia do rio Jacuí, uma drenagem costeira do Sul do Brasil. É morfologicamente mais semelhante as espécies congêneres simpátricas Oligosarcus jenynsii e O. jacuiensis, mas pode ser distinguida de ambas as espécies por possuir a região interorbital mais larga e a nadadeira peitoral mais curta e adicionalmente de O. jenynsii por possui o diâmetro orbital menor.

Phylogenetic relationships of the species and biogeography of the characid genus Oligosarcus Günther, 1864 (Ostariophysi, Characiformes, Characidae).

The characid genus Oligosarcus consists of 20 described species distributed throughout most of South American river basins below 14º south latitude. This study focus on the phylogenetic relationships of the species of Oligosarcus based on the analysis of osteological characters to provide data to discuss the biogeographic history of the genus. The analysis resulted in a single most parsimonious tree with 152 steps (CI= 0.355 and RI= 0.600). The 18 included Oligosarcus species were hierarchically organized into 17 clades. A minimal age of 15 Ma for the genus is suggested based on the putatively cladogenetic event represented by the continued shortening of the Eastern Cordillera that established the eastern boundary of the modern central Andean plateau and was responsible for cladogenesis between the common ancestor of O. schindleri + O. bolivianus versus the remaining congeners. There is a pronounced disjunction in the upland species distribution by the lowland areas of the Chaco-Pantanal basin. This indicates that upland habitats (headwater streams) are preferential habitats for a set of species including O. argenteus, O. bolivianus, O. brevioris, O. paranensis, O. perdido, O. pintoi, O. planaltinae, O. brevioris, and O. schindleri. Fragmentation of populations of O. pintoi and O. perdido are at least 2.5 Ma old, since the origin of the upper Paraguay depression clearly promoted the present-day observed disjunction in the distribution of these species. The lowland Oligosarcus species are all included in a single clade but the obtained results suggest that fragmentation of upland versus lowland components of the genus was not causally related to a single vicariant event. The available fossil record of Oligosarcus indicates that the genus already occurred in the coastal plain at about 2.3 to 1.25 Ma. The sister-group relationship between O. hepsetus and O. longirostris provided additional evidences of the so-called historical affinities between the SE Brazilian coastal plain and the Rio Iguaçu drainage basin.

Oligosarcus jacuiensis (Characiformes: Characidae), a new species from the Uruguay and Jacuí River basins, southern Brazil

The new species herein described, collected in the Jacuí and Uruguay River basins, Rio Grande do Sul, Brazil, can be distinguished from the already known species of the genus, but Oligosarcus jenynsii, O. perdido, O. acutirostris, O. solitarius and O. hepsetus, by the number of perforated lateral line scales. It shares with the first two species the absence of a premaxillary foramen, present in the last three species and differs from O. jenynsii by having a smaller orbital diameter and the tip of the pectoral fin failing to reach the pelvic-fin origin, and from O. perdido by the presence of more horizontal scale rows around the caudal peduncle.

A espécie nova descrita aqui, coletada nas bacias dos rios Jacuí e Uruguai, Rio Grande do Sul, Brasil, pode ser diferenciada das espécies já conhecidas do gênero, com exceção de Oligosarcus jenynsii, O. perdido, O. acutirostris, O. solitarius e O. hepsetus, pelo número de escamas perfuradas ao longo da linha lateral. Compartilha com as duas primeiras espécies a ausência de um forame no premaxilar, presente nas três últimas espécies e difere de O. jenynsii por possuir o diâmetro orbital menor e a extremidade da nadadeira peitoral não alcançando a origem da nadadeira pélvica e de O. perdido pela presença de mais séries horizontais de escamas ao redor do pedúnculo caudal.